A. Field of the Invention
The present invention relates generally to the field of non-contact tonometry, and more particularly to a non-contact tonometer having a non-linear pressure ramp during fluid pulse generation for increasing patient comfort.
B. Description of the Prior Art
In the operation of known non-contact tonometers for measuring intraocular pressure (IOP) of a patient's eye, an increasing force fluid pulse is discharged through a tube aimed at the eye to deform the cornea from a state of convexity, through an instantaneous state of "applanation" wherein a predetermined area of the cornea is flattened, to a state of concavity; the cornea is then allowed to return to its original convex state under natural forces. Opto-electronic means are used to continuously monitor the corneal deformation and thereby determine the moment of applanation. As shown in PRIOR ART FIG. 1, the fluid pulse is commonly generated by a piston momentarily driven by a rotary solenoid to rapidly compress air within a plenum chamber, thereby forcing air from a discharge tube in communication with the plenum chamber.
Before the introduction of small, low-priced pressure transducers capable of quickly and accurately sensing fluid pressure within the plenum chamber, the time interval required to achieve applanation was recorded and used as a correlate to IOP. Intuitively, the longer the eye had to be subjected to an increasing force fluid pulse in order to flatten the cornea, the higher the patient's IOP was. A solenoid was employed to drive the piston because energizing the solenoid coil with a constant current supply as indicated in FIG. 2 resulted in a linearly increasing pressure ramp as depicted in FIG. 3. This follows from the fact that plenum pressure is proportional to the piston velocity, and the piston velocity is proportional to the first time integal of the current supplied to the solenoid coil. Thus, a linearly increasing pressure ramp has in the past allowed for reasonably accurate calculation of IOP based on a measurement of elapsed time necessary to achieve applanation.
As pressure sensor technology became more sophisticated, non-contact tonometers included pressure transducers to measure plenum pressure directly, offering improved accuracy. Although this improvement has obviated the need for the linearly increasing pressure ramp used in older non-contact tonometers, manufacturers continue to rely exclusively on constant current for energizing the drive means to produce a fluid pulse. A recognized, but heretofore accepted, drawback of using a constant current source is the system discontinuity related to the abrupt increase in driving current from zero to its constant value, as seen in FIG. 2. Such discontinuity has dictated that a high voltage power supply be used in the instrument to provide the initial rapid rise in current, and has resulted in mechanical oscillations in the fluid pulse system.
Since the fluid pulse directed at the eye is a source of patient discomfort, and since this in turn may affect test results, an ongoing goal of those who design and manufacture non-contact tonometers has been to make the test procedure more comfortable for the patient being tested. Heretofore, efforts to improve patient comfort have been directed primarily at stopping fluid pulse generation as early as possible while nevertheless ensuring that applanation of the cornea is achieved. Examples of this approach may be found in commonly-owned U.S. patent application Ser. No. 08/659,704, now U.S. Pat. No. 5,779,633, and also in U.S. Pat. No. 5,279,300 assigned to Nidek Co., Ltd. Such prior art advancements have not involved changes in the linear nature of the fluid pulse pressure ramp.